Many types of metal articles or components, such as engine blocks, heads, oil and water pumps, brake cylinders, carburetors, transmission cases, etc., are manufactured by casting various metals or alloys, e.g. cast iron, aluminum, brass, magnesium, bronze, etc. Metal articles also are made by powdered metal techniques, in which metal powder is pressed in suitably shaped molds and then sintered at an elevated temperature to consolidate the structure, reduce porosity and impart useful strength; since this is essentially a solid diffusion process, a very high degree of dimensional accuracy is possible and large numbers of parts can be economically produced. A few examples of powdered metal articles are hydraulic pump components, gears, cams, bearings, injector bodies, levers and automotive trim.
With any of the metal casting and powdered metal processes in general use, it is typical for a varying proportion of cast articles and all powdered metal articles to have pores that extend through or partially through a wall of the articles. Because these pores can impair the usefulness of the article, such as by resulting in gaseous or fluid leakage, or preventing normal plating operations, the art of impregnating metal articles to fill the pores has developed in order that the articles can be made useful for their intended purpose.
The most effective technology for impregnation of metal articles is the "dry vacuum-pressure" method in which metal articles are placed in a closed vessel and the vessel is subjected to a vacuum (such as 2 to 10 Torr for 10 to 15 minutes) to cause air entrapped in any pores in the dry articles to escape. Liquid sealant is drawn from a reservoir into the vessel so as to cover and fill the pores of the article. The vacuum is then released and pressure is applied to the liquid sealant level (such as air at 100 psi for 10 to 15 minutes) to drive the sealant further into the pores. Following this, the excess sealant is drained, the parts rinsed, and the sealant is polymerized, i.e. cured, to fill the pores with solid material.
Another impregnation method is a "dry vacuum" process in which metal articles are placed in a closed vessel and the vessel is subjected to a vacuum (e.g. 2 to 10 Torr for 10 to 15 minutes) to cause air entrapped in any pores in the articles to escape. The articles are then submerged in or flooded by liquid sealant, so as to cover the articles and fill the pores and allowed to "soak" for 10 to 15 minutes at atmospheric pressure, following which excess sealant is drained, the articles are rinsed and the sealant is polymerized to fill the pores with solid material. An efficient dry vacuum apparatus is described in my U.S. Pat. No. 4,479,986, issued Oct. 30, 1984, the disclosure of which is incorporated herein for a more detailed description of a dry vacuum impregnation process.
Another commonly practiced technology for impregnation of metal articles is the so-called "wet vacuum" process that includes the steps of submerging the articles in a bath of liquid sealant in an enclosed impregnation tank, and then evacuating the tank to a sufficiently low vacuum (e.g. 2 to 10 Torr for 10 to 15 minutes) to enable most of the air entrapped in pores in the articles to escape so that the pores can be filled with liquid sealant. The vacuum in the impregnating tank is then released and the articles are transferred to a tank in which the excess sealant is allowed to drain off. The articles are transferred to another tank in which they are rinsed with water and/or detergent in order to wash off excess sealant. Next, the articles are transferred to a curing tank to allow the sealant to polymerize or cure to a solid material to fill the pores.
Liquid polymerizable sealants that have been proposed or commercialized to date fall into two broad classes, heat curable sealants that polymerize in the presence of air or oxygen and anaerobic sealants that polymerize in the absence of oxygen. The following lists representative patents disclosing heat curable sealants that polymerize in the presence of oxygen and summarizes their principal ingredient(s).
(1) U.S. Pat. No. 2,772,185 PA0 (2) U.K. 1,308,947 PA0 (3) U.S. Pat. No. 4,147,821 PA0 Monofunctional (meth)acrylic monomer, polyfunctional monomer (typically polyfunctional methacrylate), catalyst and inhibitor PA0 (4) U.S. Pat. No. 4,416,921 PA0 (5) U.K. 1,297,103 PA0 (6) U.S. Pat. No. 3,672,942 PA0 Monofunctional or polyfunctional acrylate and/or methacrylate monomers and free-radical imitator; washed with solvent solution of polymerization accelerator. PA0 (7) U.S. Pat. No. 3,969,552 PA0 Composition same or similar to U.S. Pat. No. 3,672,942; washed with aqueous nonionic surfactant solution. PA0 (8) U.S. Pat. No. 4,069,378 (now Re. 32,240) PA0 (9) U.S. Pat. No. 4,165,400 PA0 (10) U.S. Pat. No. 4,632,945
Unsaturated polyester with cooked-in emulsifier mixed with vinylyl monomer, e.g. 71% polyester and 29% styrene.
Methacrylic or acrylic monomer heated in absence of polymerization catalyst PA1 Acrylic monomer, azonitrile, substituted phenol and metal ion chelator PA1 Acrylate or methacrylate monomer and redox initator PA1 Composition same or similar to U.S. Pat. No. 3,672,942; sealant contains anionic or nonionic surfactant to be self-emulsifying when mixed with water. PA1 Same sealant composition disclosure as 4,069,378 PA1 Monofunctional or polyfunctional acrylate and/or methacrylate monomers, peroxy imitator, organic accelerator such as saccharin, and copper and iron compounds as co-accelerators. PA1 (1) about 25% to 40% of a polymerizable monofunctional methacrylate or acrylate monomer; PA1 (2) about 30% to 40% of a polymerizable polyfunctional dimethacrylate or diacrylate monomer ester of triethylene glycol, tripropylene glycol or tetraethylene glycol; PA1 (3) about 10% to 20% of a polymerizable polyfunctional dimethacrylate or diacrylate ester of diethylene glycol or dipropylene glycol; PA1 (4) about 5% to 15% of a water emulsifiable unsaturated polyester; PA1 (5) about 5% to 15% of a dicyclopentenyloxyethyl methacrylate or acrylate monomer; PA1 (6) 0 to about 3% of a surfactant; and the sealants are further characterized as exhibiting a weight loss of less than about 15% when subjected to a heat resistance test as hereinafter defined and weight gains of less than about 10%, 5% and 5%, respectively, when subjected to chemical resistance tests in water, brake fluid and antifreeze as hereinafter defined.
Anaerobic liquid sealants for metal articles have been promoted primarily by Loctite Corporation, and the following lists some of their patents relating to anaerobic sealants and summarizes their principal ingredient(s).
Anaerobic liquid sealants for metal articles are commercially available at the present time, some of which are formulated in accordance with one or more of patents (5)-(10) above. However, as noted in U.S. Pat. No. 4,416,921, patent (4) above, anaerobic sealants present problems since they must be supplied in two parts, one containing a catalytic polymerization accelerator that is to be added to the other part before use. The resulting mixture is liable to undergo premature polymerization, such as by contact with transition metals, and must be constantly aerated in order to prevent this. The tendency towards premature polymerization, the need for aeration, and their two part nature are said in the patent to represent drawbacks to the use of anaerobic liquid sealant compositions.
In view of the above and other disadvantageous characteristics of anaerobic liquid sealants, heat curable or thermosetting compositions are the predominant sealants of choice for almost all cast metal articles and many powdered metal articles as well. Heat curable sealants may be cured with hot water, oil, electric heating elements, hot air, etc., curing is most often practiced by submerging the impregnated articles in hot water in a suitable tank.
As noted in U.S. Pat. No. 2,772,185, (1) above, and U.K. Patent 1,308,947, (2) above, some of the early heat curable sealants consisted of unsaturated polyester mixed with a polymerizable monomer such as styrene, but these compositions presented problems such as high viscosity and the necessity to wash with solvents that inhibited their use; also, the composition proposed by (1) above, is particularly difficult to produce since it proposes a polyester with a cooked in emulsifier and monomer. The U.K. patent, (2) above, proposes a methacrylic or acrylic ester monomer or a mixture of such esters, as a sealant which is cured in the absence of a polymerization catalyst; the cure times described in the patent are quite long, one hour for curing being described in the patent. I do not know of any commercial use of sealants according to either U.S. Pat. No. 2,772,185 or U.K. 1,308,947 at the present time and it is believed that the sealants described in these two patents cannot meet the requirements of current industry practice.
Heat curable sealants said to be formulated according to U.S. Pat. No. 4,147,821 (3) above, have been and are currently sold commercially by the assignee of this patent and others, and sealants said to be according to U.S. Pat. No. 4,416,921 (4) above, are sold commercially by Loctite Corporation. These types of sealants are of interest to end users since they can be cured at relatively low temperatures in relatively short time periods, such as by curing with hot water in a temperature range of about 85.degree. C. to 90.degree. C. for a period of about 4 minutes to 15 minutes. Rapid curing at moderate temperatures facilitates impregnation of metal articles and reduces the time required for processing the articles. However, these sealants, while adequate, do not provide high performance characteristics presently sought by many end users. Among the disadvantages of current commercial heat curable liquid sealants are (i) relatively high heat loss, often expressed as low heat resistance and (ii) relatively poor chemical resistance, particularly to fluids such as brake fluid, antifreeze solutions and deionized water, and (iii) poor sealability. There is, therefore, a need for a heat curable liquid sealant for cast metal and powdered metal articles that provides better heat resistance, better chemical resistance and better overall sealability characteristics than currently-available sealants, and this was the impetus for the development effort that culminated in the present invention.
A principal object of this invention is to develop a heat curable liquid sealant for cast and powdered metal articles that provides improved heat resistance, chemical resistance and sealability as compared to presently-available sealants. Another principal object is to develop sealants with the foregoing advantages while retaining the characteristic of being curable at moderate temperatures and relatively short time periods. Another main object is to provide new heat curable liquid sealants that can be used to impregnate porous cast metal and powdered metal articles with impregnating apparatus and methods now in general use, i.e. that do not require special equipment or unusual processing steps. A further main object is to provide new compositions for liquid sealants that can be readily formulated by simple mixing of ingredients without employing reaction vessels or prolonged processing. Another principal object is to provide liquid sealants that have a viscosity within a range suitable for effective impregnation of metal articles. Yet another is to develop new liquid sealants that exhibit excellent sealability of the pores of metal articles. Still another main object is to accomplish all of the foregoing objectives and yet provide new sealants of moderate cost so as to enable end users to realize enhanced performance benefits without additional expense. These and other objects will become apparent in the detailed description that follows.